Shift in collagen type as an early response to induction of the metanephric mesenchyme.

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Abstract
Conversion of the nephrogenic mesenchyme into epithelial tubules requires an inductive stimulus from the ureter bud. Immunofluoresence techniques showed that the undifferentiated mesenchyme before induction expresses uniformly type I and type III collagens. Induction in vivo and in vitro leads to a loss of these proteins and to the appearance of basement membrane components including type IV collagen. This change correlates spatially and temporally with the determination of the mesenchyme and precedes any morphological events. During morphogenesis, type IV collagen concentrates at the borders of the developing tubular structures where, by EM, a thin, often discontinuous basal lamina covered the 1st pretubular cell aggregates. Subsequently, the differentiating tubules were surrounded by a well-developed basal lamina. No loss of the interstitial collagens was seen in the metanephric mesenchyme when brought into contact with noninducing tissues or when cultures alone. Similar observations were made with nonnephrogenic mesenchyme (salivary, lung) when exposed to various heterotypic tissues which induce tubules in the nephrogenic mesenchyme. The sequential shift in the composition of the extracellular matrix from an interstitial mesenchymal type to a differentiated epithelial type is the 1st detectable response of the nephrogenic mesenchyme to the tubule-inducing signal.